A Manual Of Photography

EARLY RESEARCHES ON THE SOLAR RAYS. 7 Those appear to be the more important researches in connection with this particular section of the inquiry, until tire correction of the statements of Saussure were published by Dr. Daubeny, who satisfactorily proved that light, the luminous power as represented by the yellow rays, as distinguished from the chemical power or blue rays, was the most active in producing the decomposition of carbonic acid by the leaves of plants; and these results were confirmed by my own researches pub- lished in the Reports of the British Association.* Although these influences upon living organisms are directly connected with the chemical actions of the solar rays, the phænomena, being of a very complicated character, require an enlarged series of researches before any correct deductions approaching to the generality of a law can be made. The earliest recorded attempts at fixing the images of the camera obscura by the chemical influence of light, are those of Wedgwood and Davy, published in the Journal of the Royal Institution of Great Britain, in June, 1802. Neither of these experimentalists succeeded in producing a preparation of suffi- cient sensitiveness to receive any impression from the sub- dued light of the camera obscura. By the solar microscope, when the prepared paper was placed very near the lens, Sir II. Davy procured a faint image of the object therein ; but being unacquainted with any method of preventing the further action of light on the picture, which is, of course, necessary to secure the impression, the pursuit of the subject was abandoned. Wedgwood was certainly the first person who made any attempts to use the sunbeam for delineating the objects through which it permeated: it is therefore necessary that some more particular account should be given of his processes. In 1802 he published a paper in the Journal of the Royal Institution, under the following title : " An Account of a Method of Copying Paintings upon Glass, and of making Profiles by the Agency of Light upon Nitrate of Silver ; with Observations by II. Davy.’ From this communication the following extracts, containing the more important indications, are made. “ White paper, or white leather, moistened with solution of nitrate of silver, undergoes no change when kept in a dark place, but, on being exposed to the daylight, it speedily changes colour, and after passing through different sirades of grey and brown becomes at length nearly black. The alterations of colour’ take place more speedily in proportion as the light is more intense. In the direct beam of the sun, two or three * Report of Seventeenth Meeting, 1847, p. 17.